Cardiac hypertrophic remodeling underlies a large component of the morbidity and mortality of heart disease. It affects nearly 10% of the world's population given the high prevalence of hypertension and hypertrophy that evolves with it. Cyclic guanosine-3', 5'-monophosphate (cGMP) is an intracellular second messenger which controls diverse cell physiology. Accumulating evidence shows that enhanced synthesis of cGMP by nitric oxide or natriuretic peptides negatively regulate cardiac hypertrophy development. We recently found that blocking cGMP degradation with phospho- diesterase5A (PDE5A) inhibitor have potent anti-hypertrophic and anti-remodeling effects. One major downstream mediator of cGMP is cGMP-dependent protein kinase (cGK). It remains unclear how cGMP and its downstream cGK regulate intra-cellular signaling network, resulting in such physiological impact on the heart. PDE5A inhibitors are currently widely used to treat erectile dysfunction and pulmonary hypertension. Reports from us and others supporting cardiac benefits from cGMP enhancement by PDE5A inhibitors have raised substantial interest for these drugs in treatment of human heart disease. To clarify the underlying mechanism of this signaling takes on more prominence now that the NIH is expected to initiate a clinical trial of PDE5A inhibitor in heart failure patients. Our preliminary data suggest that cGMP- activated cGK type I1 (cGKI1) targets two different molecules to exert beneficial effects over stressed or diseased heart, depending on its intracellular location at different stages of cardiac remodeling. cGKI1 blunts Gq signaling by activating its inhibitory protein named regulator of G-protein coupled signaling 2 (RGS2) at the sarcolemmal membrane in the early phase of hypertrophy development, whereas in the late phase (failure), it up-regulates PGC-11 (peroxisome proliferators-activated receptor 3 coactivator 1-1), a master regulator of mitochondrial biogenesis and function possibly in the nucleus. The research in this proposal aims to provide critical information on the mechanisms of this regulation in two aims by utilizing animals deficient in RGS2 (conventional and inducible conditional) and animals harboring mutant cGKI1 in which critical protein- protein interaction site (leucine zipper motif) is disrupted. The first aim will examine the cardio- protective mechanism of GKI1 in the early hypertrophy development phase. The second aim will examine the role and the mechanism of PGC-11 regulation by cGKI1, and its impact on mitochondrial biogenesis/ function in the late phase of cardiac failure. The successful completion of these studies will greatly expand our understanding of the role for cGMP signaling in diseased hearts, and inform clinical trials testing cGMP signaling-enhancing drugs such as PDE5A inhibitors for treating heart disease.